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When I read about how to select wire gauge, it talks about length of run and amps. This part I understand. However, I'm installing some LED light strips. These lights will be powered by a 24V output power supply.

These LED bars say you can connect 10 or 16 feet of them in a chain, depending on how many connections. The real limit is 10 connections. There are 1 ft and 2 ft bars, hence 10 and 16 being different. Basically, they are calling out that the real limit is 10 connections.

Since the real limit is the number of connections, I think I can inject a 20 foot length of wire between bars. In trying to calculate the wire gauge for this segment, I use 5 Amps and 20 feet, which gives me a conservatively sized 14 AWG wire.

AND MY QUESTION:

I understand the math behind the 14 AWG wire as cited above. However, once the run of LED bars starts, each segment is then connected with a 22 gauge wire. And, if I didn't need the extra length, the wire they ship with, for connecting the power supply to the head of the light bars, is also 22 gauge.

Why does the math call for a larger gauge wire, while the appliance connecting to it has much smaller wires? Aren't those smaller wires to the device carrying the same current? Why can they be smaller?

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  • if you are adding this wire to an exiting circuit, what is the size of the breaker in your electic panel protecting the circuit? AFIK this would be either 15 A or 20 A. The wire used (except for wire within a fixture) is sized for the breaker 14 AWG for a 15 A breaker and 12 AWG for a 20 A breaker. Commented Nov 19, 2022 at 23:53
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    You need to clarify if the LED strips that you are installing are the type that operate off of a 12V DC power source. Answers so far here and above comment are aimed at AC mains wiring which would only really apply to the AC wiring that feeds the AC Mains -> 12V DC LED power supplies for LED strips.
    – Michael Karas
    Commented Nov 20, 2022 at 0:00
  • @MichaelKaras I edited to make the clarification you called for
    – chad
    Commented Nov 22, 2022 at 20:13
  • Why does # connections matter?? Total circuit resistance and total light power matter. Connection count shouldn't?
    – jay613
    Commented Nov 22, 2022 at 21:19
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    Well I suppose if the document says to limit the number of connectors, maybe they aren't very good connectors? I mean, to say "all connections introduce loss" is a little abstract. The connectors in a system should not be a significant source of loss or resistance ... if they are, the system should be designed with better connectors.
    – jay613
    Commented Nov 23, 2022 at 1:28

2 Answers 2

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Low voltage DC is dramatically more sensitive to voltage drop than AC mains.

Like 10 times more sensitive for 12V vs 120V. So yeah, 12V at 5A at 20’ round trip is about 3% voltage drop for #12 wire.

You also have the internal resistance of the wires or traces in the LED strip. You cannot just assume they have infinite ampacity.

You must compute the voltage drop of each wire segment separately and add them up. Remember to figure round trip if doing it by ohms per foot and math.

To improve the situation, you can use 24 volt LED strips.

#22 is plain too small for 5A at any distance. 3A max I would think. Good for one strip not two.

Now where do you buy #14 and #12 cable cheaply? Home stores. Use the stuff intended for AC power. Romex indoors, UF outdoors or THHN. It's dirt cheap compared to #12 hobby or vehicle wire. Use wire nuts to splice it down, e.g. an orange wire nut for #12 to #18 and then blue ones for #18 to #22.

What if the voltage drop calc says you need larger? Do not use #10 copper, too costly. Next step is #6 alumimum. Splice that with "accessory ground bars" intended for service panels, cut up. They are made of aluminum and take either aluminum or copper in any port (not mixed). This isn't legal for mains AC usage, but for your low-voltage DC project, you can get away with putting 2+ copper wires in 1 port.

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  • “Romex indoors, UF outdoors or THHN. It's dirt cheap compared to #12 hobby or vehicle wire.” Have you actually priced NM lately? HD wants significantly more per foot in 100’ quantity for both 14/2 and 12/2 vs. Ancor tinned, stranded two-conductor cable of the same gauge and length (even worse for shorter lengths). Probably partly due to needing extra metal for the dedicated ground wire, and partly due to supply and demand.
    – nobody
    Commented Nov 24, 2022 at 15:26
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I run a lot of LEDs for decoration, and I haven't used anything larger than 16ga wire - and that was for power to a distribution board that branched out 4 different ways. My longest power injection cable is about 45', it's 18ga. Voltage drops over that distance from 12v to about 10.5v which is still more than adequate. You'll notice vdrop problems best on bright all-white settings which pull the most current since all the RGB components are on high brightness -- this will show up as the nodes at end of the string noticeably dimmer than the rest.

Voltage injection is necessary because as you've seen, the wires between nodes are 22ga and the copper traces on LED strips are similarly extremely thin. Neither carry much power, and they're not designed to. Using thicker wires/traces would make the LEDs way bulkier and more expensive.

You can do a lot of testing and math (in fact, this is encouraged because it's a fun part of the hobby) but several LED community members have already calculated tables and have rules-of-thumb for most situations, and none of them recommend 14ga or 12ga wire.

LED control signal is one-way, but voltage injection is bi-directional. So if you need injection after 5 strings and you inject it at the beginning and at 10 strings, it'll drive LEDs from 5-15 strings (+-5 from the injection site). So if you have multiple injection points, they don't need to be as close together as they would if they were at the ends.

Calculations and testing can help you select a proper fuse, which is recommended to protect against shorts or wire damage which can cause wild currents.

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